I improved mine significantly without change the waste or cutting the pipe shorter...
I found that the 'S' bend pipe dropped slightly in the middle meaning the second half had to travel up hill thus reducing flow rate. Easy solution, zip tied the middle of the pipe up to the bodywork (happens to be some triangular gaps to wrap on to above) to create a constant slope down to the waste tank. Much quicker flow and no smell.
Thanks for the tips. Although not covered by the All In warranty, my local dealer acted out of goodwill and cleared the blockage with some careful use of a welding wire. It has never run so freely and I will follow the advice to use an unblocking agent at least annually.
Hats off to Citygate VWVC, Wooburn Green and to Master Tech, Stewart Warner in particular